1. Field of the Invention
The present invention relates to an application head and a droplet applying apparatus applying a predetermined amount of liquid (droplet) such as resin or adhesive to an object to be coated.
2. Description of the Related Art
In the electronic components industry, droplet applying apparatuses are required to apply a droplet to an object to be coated at high speed and an accurate application amount (discharge amount). At the same time, since a droplet remaining at a tip of a nozzle clogs the nozzle or causes a discharged droplet to attach to the nozzle again, management of the droplet applying apparatuses is also an important problem. Arrangement of a plurality of application heads in a droplet applying apparatus is conceived as a technique of speed-up and, therefore, downsizing and thinning of the application heads are also required.
Since it is difficult to stably discharge an already required accurate application amount with a pressure air type applying apparatus having a relatively simple structure, systems with a mechanical structure having less variation are under study. Application of a plunger type applying apparatus is considered as one of those systems.
Japanese Laid-Open Patent Publication No. 2004-019593 (first patent document) describes a reciprocating pump having a plunger reciprocating in a pump chamber formed in a pump head, a first check valve disposed in a vicinity of a port of a suction flow passage as a suction valve, and a second check valve disposed in a vicinity of a port of a discharge flow passage as a discharge valve, wherein a suction pipe and a discharge pipe can be connected to the upstream side of the suction valve and the downstream side of the discharge valve, respectively. The check valves form a sub-flow passage generating a pressure difference in the axial direction of a main valve element by a fluid flow, and a sub-valve element is disposed in a sub-valve chamber.
An object of International Publication No. 2007-046495 (second patent document) is to provide a liquid material ejector that has a structure without any unnecessary projection and spread of the ejector in the horizontal direction with respect to the advancing/retracting direction of a plunger while being able to handle a liquid material of any viscosity and that enables linkage of a plurality of ejectors. The liquid material ejector is made up of a liquid material supply opening for supplying a liquid material, a nozzle for ejecting the liquid material, a valve block having a weighing hole to be filled with ejected liquid material and a liquid material supply channel communicating with the liquid material supply opening, a switching valve having a first channel for allowing communication between the weighing hole and the liquid material supply channel and a second channel for allowing communication between the weighing hole and the nozzle, a plunger advancing/retracting in the weighing hole, a plunger driving unit for driving the plunger, a valve driving unit for actuating the switching valve, and a transmitting unit transmitting a drive force from the valve driving unit to the switching valve. The liquid material ejector is characterized in that the plunger driving unit, the valve driving unit, and the valve block are arranged sequentially in the longitudinal direction.
In the first patent document, the respective check valves (non-return valves) are disposed on the suction side and the discharge side and the pump chamber (cavity) and the plunger are provided between the non-return valves. Assuming that the cavity is filled with liquid and that the liquid is always supplied to the suction side at a predetermined pressure, the liquid is supplied from the suction side when the volume of the cavity increases due to plunger operation and the liquid is discharged from a discharge opening when the volume of the cavity decreases due to plunger operation. In the case of the first patent document, the sub-flow passage is disposed for improving the response of the non-return valves. However, a method using the non-return valves cannot realize the accurate application amount (discharge amount) required in the electronic components industry. Specifically, with the processing capacity of the non-return valve system, it is difficult to achieve one shot per one second or less than one second and it is difficult to achieve highly-accurate discharge (with variation of 0.5% or less) at a minute amount (e.g., 0.005 ml or less).
In the second patent document, the switching valve rotatable is included, and a concave groove of the switching valve acts as a supply side and a through hole of the switching valve acts as a discharge side, and are switched to fill the weighing hole (cavity) with liquid so that movement of a plunger rod projecting into the cavity changes the volume in the cavity to discharge the liquid. It is described that this configuration enables reduction in size of the application head and sequential arrangement of a multiplicity of nozzles as the effects of the invention of the second patent document. However, in the case of the second patent document, because of the structure causing the concave groove and the through-hole of single switching valve to act as the supply side and the discharge side, respectively, a state of the switching valve must be selected from only two types, i.e., either communication only on the supply side or communication only on the discharge side, which is inconvenient since air removable and washing of a liquid passage (flow passage) are made difficult.